Why
Because my Jenkins setup sometimes starts containers and forgets about them. Either it thinks it failed to start one, or the container itself has trouble starting. Either way, I'm left with containers that are running, trying to connect to Jenkins in vain, forever. The proper way to fix this is probably to have the containers timeout at some point, but that mechanism is broken.

Anyway, the fix I have is a true hack: find containers that have been up more than 2 days and kill them. None of our jobs should run for more than about a day, so this is a safe limit. Here is a bash script to do this: